1. Field of the Invention
The present invention relates to an illumination device which uses a laser as a light source, and more particularly to an illumination device suitable for an exposure device used in the fabrication of an integrated circuit.
2. Related Background Art
As a light source of the exposure device used in the fabrication of integrated circuits, an ultra-high pressure mercury vapor lamp has been mainly used in the past.
Recently, integration density of the integrated circuit has been increasing rapidly and a higher line width accuracy is required.
Accordingly, instead of the ultra-high pressure mercury vapor lamp, a short wavelength, high power laser such as an excimer laser is used as a light source of the exposure device.
However, when a narrow wavelength band laser is used, such as one which uses injection locking, speckle is generated by interference, and formation of a fine pattern is impeded.
In order to prevent the generation of speckle, an illumination device as shown in FIG. 2 has been used in the past.
In FIG. 2, a laser beam LB emitted from a laser light source 10 passes through lenses 12 and 14 and is directed to a deflection mirror or vibration mirror 16 where it is deflected, and then passes through a lens 18 and forms a spot at a position PA.
The laser beam LB focused by a condenser lens 20, is directed to a reticle R, passes therethrough and is applied to a projection optical system 22 which is telecentric on both an object side and an image side.
The projection optical system 22 has a front group of projection lenses 22A and a rear group of projection lenses 22B. The laser beam LB which has passed through the projection optical system 22 is directed to a wafer W. Thus, a circuit pattern on the reticle R is projected onto the wafer W.
The illumination device will now be explained in further detail. The laser beam LB is expanded in diameter by the lenses 12 and 14 and directed to the vibration mirror 16. The vibration mirror 16 is vibrated around a rotation axis, as shown by an arrow, during the exposure so that an expanded secondary light source is formed at the position PA.
The image of the secondary light source is focused to a pupil position 22P in the projection optical system 22 by the condenser lens 20 and the front group of projection lenses 22A in the projection optical system 22.
Since the light from the respective focus points of the secondary light source is considered to be incoherent, no speckle should be generated on the wafer W.
However, in the prior art illumination device, light intensity distribution on the reticle R and the wafer W is not uniform but is a gaussian distribution or close thereto.
In accordance with an illumination optical arrangement disclosed in U.S. Pat. No. 4,619,508 assigned to the assignee of the present invention, in order to secure uniformity of the light intensity distribution, a collimator lens and a flyeye lens (not shown) are inserted between the secondary light source at the position PA and the condenser lens 20, and the light source images by the flyeye lens are focused to the pupil position 22P of the projection optical system 22.
However, the light at the pupil position 22P does not exhibit sufficient in coherency even if the vibration mirror 16 is vibrated, and interference generates speckle on the wafer W.